Skiers love the Great Salt Lake effect, but it is a serious thorn in the side of Utah weather forecasters. So much so that we call it the "dreaded lake effect" or DLE for short.
We've just submitted a paper entitled Great Salt Lake-Effect Precipitation: Observed Frequency, Characteristics, and Associated Environmental Factors that presents a revised climatology that we hope will improve both understanding and prediction. The project was spearheaded by Trevor Alcott, a graduate student here at the University of Utah, and includes collaboration with Neil Laird, a professor at Hobart and William Smith Colleges in upstate New York. Neil's students Benjamin Albright and Jessica Popp performed the initial event identification for the climatology.
The climatology is based on 149 lake-effect events that were identified over 13 seasons. Skiers will be interested to learn that the event frequency is greatest in the fall (Oct–Nov) and spring (early Apr) with a mid-winter minimum. This contrasts with the mid-winter maximum identified in an earlier study by my group (Steenburgh et al. 2000), which was based on a much shorter period of record and a small number of events.
|Number of lake-effect events by half month|
(Alcott et al. 2012)
One of the more interesting aspects of the Great Salt Lake effect is its diurnal (i.e., day to night) modulation. Events tend to trigger after sunset and dissipate after sunrise. This diurnal modulation is strongest in the spring (Mar–May) and weakest in the winter (Dec–Feb).
|Number of days with lake effect by time of day [UTC|
and Local Standard Time indicated (Alcott et al. 2012)]
For skiers looking for bluebird days, this is a great result. Snowfall at night, clearing skies during the day. Of course, as we will discuss in a few weeks when we submit a second paper on this subject, the amount of snow produced by lake-effect is nowhere near as large or as important as suggested by conventional wisdom, hype, and ski brochures.
As many people recognize, the movement of a cold airmass over the relatively warm lake surface is a necessary but not sufficient condition for lake-effect. One of the more interesting findings of the study is that lake-effect events during winter more frequently occur during periods of weaker differences in temperature between the lake and the atmosphere at 700 mb (10,000 ft) (a.k.a. the lake-700 mb temperature difference, which is commonly used to assess instability over the lake). Or, alternatively, lake-effect in the spring and fall typically requires a larger difference in temperature between the lake and the atmosphere at 700 mb.
The paper identifies some of the shortcomings of existing forecast techniques, which are based primarily on the difference between the lake temperature and the 700-mb temperature. Instead, we propose a new approach based on a seasonally varying lake-700 mb temperature difference threshold (which we call ΔTexcess) and low-level relative humidity. The historical likelihood of lake-effect based on these two variables is shown below.
|Fraction (%) of soundings with lake effect as a|
function of ΔTexcess and low-level relative humidity
(Alcott et al. 2012)
In the paper, we also discuss how wind direction and mid-level relative humidity affect the likelihood and coverage of lake-effect precipitation.
These results will hopefully improve lake-effect forecasting. I suspect that the primary benefit will be a reduction of false alarms, especially events where lake-effect is forecast but doesn't occur. That being said, this study is not a panacea. We have more work to do and I suspect forecasters will continue to call it the dreaded lake effect (or something saltier) in the coming years.
Stay tuned for a second paper on lake-effect that should prove even more interesting for skiers.